The agreement shall specify, in addition to those things required by section 13-804, (1) the date upon which the initial board is to organize, (2) the geographic boundaries or limits of the districts into which the joint authority shall be divided, of which there may be no more than five, from which the members of the initial board shall be appointed and from which their successors shall be elected, (3) the number of board members to be initially appointed, and thereafter elected, from each district designated pursuant to subdivision (2) of this section, and (4) the method by which the five members of the initial board shall be appointed and the duration of their respective terms of office. The limits of each district may be changed only upon the affirmative vote of a majority of the whole membership of the board. Each member of the board shall be a registered voter and reside within the district from which he or she is appointed or elected. The terms of office of the members of the initial board shall expire at such time as their successors shall have been elected and qualified pursuant to section 32-549. Vacancies on the board, other than those resulting from expiration of a term of office, may be filled by a majority vote of the remaining members of the board. Any member so appointed shall serve until a successor is elected at the next general election to serve the unexpired portion of the term if any.